It is well known to employ the magnetic field of a member or members associated with a roadway to guide vehicles having an automatic steering system. It is also known to employ a guidance wire system associated with a roadway that emits a radio signal which is picked up by an antenna on a vehicle and processed by an onboard computer that controls vehicle steering.
Although autoguidance systems employing radio frequency wires are satisfactory in many respects, they do have significant disadvantages. For example, power must be supplied to the wire, not always feasible in remote areas. Furthermore, RF wire guide systems require use of a continuous wire loop in or on the roadway and each loop must be associated with its own power supply source or sources. Such arrangements are quite expensive and prone to damage; for example, a lightning strike at or near an RF wire installation can knock out the system.
Magnetic vehicle autoguidance systems also have a number of limitations. According to this approach, either spaced magnetic field emitters or elongated elements such as magnetic tape are positioned at the roadway and the magnetic field or fields emitted thereby are sensed and a resultant signal produced to control a vehicle automatic steering system.
In order to sense the magnetic field of the tape a sensor must be placed in very close proximity thereto. Not only is the magnetic field of the tape relatively weak, the sensor picks up an ambient magnetic field including that produced by the earth itself. This problem becomes even more aggravated if bridges or other metal structures are in the vicinity.
Because of road irregularities or other factors, it is often difficult or even impossible to maintain the position of a sensor on a vehicle very close to the road surface, as presently required to pick up weak magnetic signals produced by available magnetic tapes and distinguish them from ambient magnetic fields.
Wide magnetic tapes has been utilized in an attempt to strengthen the magnetic guidance field, however such an approach is not only expensive but provides only a partial solution to the problem of ambient or extraneous magnetic fields interfering with operation of the autoguidance system.